(a) Field of the Invention
The present invention relates to a method and a system for measuring the amplitude and phase angle of harmonics in a periodic signal by utilizing a tunable tracking filter which is connected to receive the periodic signal having a frequency within a predictable range. The tunable tracking filter may be a switch capacitor filter or an analog-type filter.
(b) Description of Prior Art
As far back as the 1920's, power system harmonics were an important matter under investigation by power utilities and telephone companies. Early problems were mainly related to low-order current harmonics generated by transformers and induction motors. The most successful effort to reduce these harmonics utilized the transformer connections. For example, delta transformer connections act as a blocking filter for the 3rd, 6th, 9th, etc. harmonics. Today, the proliferation of non-linear loads, created mostly by static power converters, has increased the level of power harmonics and represents a growing source of problems afflicting other customer loads and the power network itself.
The sources of harmonics are multiple: transformers, induction motors, arc furnaces and static power converters for electrochemical processes, dc and ac drives, HVDC transmission and var control. Most of these sources act as non-linear loads affecting primarily the current waveform. Some common cases of distortion are as follows:
(a) rectangular shapes due to the rectifying process;
(b) stair-step shapes generated in the synthesis of sinusoidal waveform;
(c) high frequency pulses caused by direct ac to ac conversion involving constant switching from one phase to another.
Even in the presence of pure sine wave voltages, the effect of current harmonics produced by a specific load will affect the distribution system. Considering the power system example of FIG. 1, one can see that the distorted currents, flowing through the characteristic impedance of the transmission line, will generate voltage harmonics. The other customer loads connected to the same network will suffer from this electrical pollution.
The evaluation of the effects of harmonics on power systems is a complex task. Some valuable assessments are given in an article of the I.E.E.E., Working Group on Power System Harmonics, 84 EH0221-2PWR, New York, 1984, which also list many interesting studies on the subject. Moreover, utilities are very concerned about the growing level of system harmonics and about the application of measures to temper this phenomenon. Many measurement techniques have been suggested in the prior art literature.
Phase angle and amplitude measurement of harmonics can be classified according to two procedures. The first method performs selective null detection in conjunction with a wave analyzer. The second method is based on Fourier series (orthogonality). Both procedures yield acceptable results if the instruments used are properly designed and calibrated. However, their implementation on a large scale could suffer from drawbacks like the high overall cost, the sensitivity of critical components and the time needed to process the information.
Many methods are known for measuring electrical parameters such as active power and apparent voltamperes. The most common are: electrodynamometers, thermoinstruments, electronic multipliers, Hall effect transducers, fast Fourier transform systems, dedicated distortion analyzers, and frequency selective voltmeters. However, most of these methods fail to provide an easy measurement of the displacement power factor. For example, in single phase, single frequency, the measurement can only be performed by using a high-Q tuned filter or with the help of elaborate methods. Both analog and digital techniques currently used to extract the fundamental component of an electrical signal are plagued with severe shortcomings. The analog techniques involve serious delays due to the low-pass filters required while digital ones suffer due to the considerable computing time required by FFT techniques. Therefore, these methods are ill-suited to measure the power displacement factor of variable frequency power conditions, where both current and voltage may be heavily distorted, and where the frequency may well vary between 1 Hz and 200 Hz.